I had an episode of rapid heart beat one time when I was on a 30-day juice fast. It happened about 15 days in. I got a little scared and was going to break the fast, but I did some reading and found that in one of my books (I think it was The Miracle of Fast) the author said that that condition was due to drugs leaving the system. If you have done a lot of drugs in your life , either over-the-counter, prescription, or illegal, they are stored in the body and could cause that conditions when fasting.